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Old 11-10-2002, 01:41 AM   #11
jim_howard_pdx
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Location: Portland Oregon
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Kevin,

Your point is well considered. Let's review some basics. The engine cannot build more horsepower than the amount of air and fuel it can pack into the combustion chamber relative to the compression (either static or dynamic).

As a result of this principle, Yeats built an Intake valve size to dominate the NASCAR field. He sized the intake valve to build the power he needed, then ran the best possible exhaust valve given the geometry of the cylinder, valves, push rods, spark plug, and so forth.

Also you may know that the port pocket of any valve cannot be larger than 80% of the valve size. We need this much meat around the valve seat, or it will collapse and this is really an ugly sight.

The feeling is that the exhaust is easier to move than the intake charge. And this is basically correct until rpm goes up, the exhaust backpressure rises, the valves are now moving faster than the intake charge moves!

Ha, I shared a secret with you Kevin that I was not going to mention just to see if anyone out here has built top end power engines like those used at Bonneville.

The valve opening and closing at 9,000 RPM is so fast, and the fuel charge is so dense and thick that it is somewhat like pouring molasses into a jar. I am oversimplifying, but I know the point is not lost on you. The air fuel mixture is a liquid, the valve event becomes faster and faster and less air and fuel can be packed in. It is just simple physics.

That is why most all high rpm engines are supercharged or turbo charged, not because they by themselves push more air and fuel into the engine, but because the provide a positive pressure behind the air and fuel at the intake pocket that with the proper overlap will generate good cylinder pressure that atmospheric pressure could not accomplish.

So enlarging the exhaust valve to the optimum flow is really not possible. To solve this, several european manufacturers explored 3 valve per cylinder heads in the 60's and 70's.

The first 3 valve cylinder configuration I saw tried to rectify this by using one large intake valve and two small exhaust valves that yielded much better exhaust flow figures. A 2V race engine with maximum porting typically delivers 80% of the intake's flow at the exhaust end. But this three valve design delivered nearly 95% flow and the engine built great power.

Within one year 4V engines became the norm. The crossflow of the 4V over-comes any slight benefit the 3V engine might have. It drags an amazing amount of intake charge at very nominal dilution. This is why my Honda VTEC engine makes 90 horsepower per liter and the typical early 351 Lightening engines make 66.37 horsepower per liter. 4V is simply a superior valve design to enhance cylinder filling and cylinder evacuation.

Going back to your point, you mention that too large an exhaust valve and port or too large an exhaust system would reduce exhaust flow and create lower torque and more dilution.

Yes it is possible to have too large a primary tube, and have overly large exhaust pipes. Large is not necessarily good in exhaust systems. But in your engine Kevin, your primaries, collectors and exhaust pipes seem PERFECT for the power band you are working to achieve.

Just see if you can build an insert to spin the exhaust in your collector, and run a spin tech muffler. Don't be concerned if your peak horsepower goes down 2 to 4 horsepower, because you are after the spin throughout the entire rpm range and your ets should go down just a little bit from the extra scavanging, even with slightly lower peak horsepower. The torque under the curve will expand, and this will be worth the trade-off.

Kevin did hear about exhaust "ringing" before my speed secret. This is fascinating physics I did not learn until I hear a 427 SC Competition Cobra. I was shocked by how loud the exhaust was. The owner explained that the side pipes spun the exhaust but did not muffle it. I asked why spin it, and he filled me in on spin tech's work.

Then I read Carroll Shellby's recount of the SC Cobras, and he mentioned it in greater detail. The SC Cobras had basically the same engine. same cam, same carbs, same intake, but produced nearly 500 flywheel horsepower. Most of this due to the spin techs and extra balancing, blueprinting, and equalization of the carburation. They were using either twin holleys, or down draft webbers. The webbers were tuned for maximum midrange torque for twisty courses. The twin holleys worked better on long straightaway tracks.

It is fascinating that the exhaust slams side to side inside the primaries, collectors, and exhaust tubes with greater horizontal velocity than its linear velocity. Chevy picked up nearly 25 horsepower in its 454 Rat Race Heads when they brought out the D port design. They saw what Valley Heads did with the Boss 302 heads and 351 4V heads with their ultra cool raised D port plates and they pretty much copied this into their race head. Most racers used the old rectangular ports because the better oval ports had really aweful low rpm torque (just like the Cleveland 4V engine.) Now the new heads flowed like the ovals with torque like the rectangular heads.

We discovered that the D ports actually spin the exhaust as it comes into the primaries. We used clear plastic tubes, and colored smoke with strobe lights to see the effect. The spin was present in almost every rpm range, but highest at the cam's maximum torque peak which we were counting upon.

In fact, you did not want a d port shaped primary tube on the header, because it would kill the spin created by the D port. So the exhaust port shape made alot of the ringing energy diminish and their exhaust velocity was increased. This netted the extra 25 horsepower!!!!

Within the last 20 years, it is correctly assumed that there is a high end for the cam overlap which should not be exceeded. The higher the overlap, the worse the dilution.

That is why Kaase, the guy that created the 429 Aluminum Cobra Jet Head rarely uses a cam with over 290 degress duration at .050 lift. He would rather run .780 lift to get the dynamic compression up with minimum overlap, than dilute the intake charge and waste the potential torque.

But the 429 allows very large valves, very high lift, incredible cfm flow, and great valve geometry. So he can run less overlap to generate 800 horsepower on gasoline. If we small block guys want 800 hp on gasoline, we have to build in more overlap to get the cylinder pressuse we need.

Good job Kevin. I think you have done the best in terms of digesting all this detail.

Let me know if you experiment with spinning the exhaust in your collectors. I think you will be surprised with the results.
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